Vented, gas-fired appliances relying on natural draft for the removal of products of combustion are equipped with a draft hood which isolates the combustion chamber from excessive updraft or backdraft in the vent. The conduit between the draft hood relief opening and the outside is permanently open and therefore causes heat loss, especially during cold and windy weather.
To reduce the heat loss automatic vent dampers are installed between the draft hood and vent connector. The heat savings are optimized by restricting the air flow in the vent to a safe minimum.
During standby the safe minimum is a sufficient air flow to remove the vent gases of the pilot flame, if so equipped, and to provide some air change in the vent to prevent excess condensation.
During the operating phase of the appliance the safe minimum is a sufficient air flow to remove the products of combustion while retaining heated room air, regardless of the varying draft in the vent.
Thermally actuated vent dampers have been designed to perform these vent restrictions.